Mitigation of RF Sheaths Impurities Generation by Structural Optimization of ICRH Antenna for EAST

Ion Cyclotron Resonant Heating (ICRH) and Current Drive (CD) are effective methods of plasma heating and confinement in tokamaks. The generation of impurities associated with ICRH & CD is one of the critical issues. Studies indicated that the fluxes of ions accelerated in the radio frequency (RF)-driven sheaths closed to antenna give rise to the parasitic impurity sputtering, arcs and the hot spots on the antenna surface, which will deteriorate the plasma confinement and even cause safety issues such as breakdown of ICRH system. This work reveals that the imbalance of magnetic flux through flux surface controlled by the phase composition and the unsatisfactory structures of antenna components can cause large parallel components of electric field (E//), which in turn lead to the formation of RF-driven sheaths. Aiming to mitigate the RF sheaths impurities of the ICRH antenna in Experimental Advanced Superconducting Tokamak (EAST), an alternative plasma load was used during the modeling study to obtain the near-field characteristics. Based on the analysis of the field properties near the antenna, several designs that can alleviate impurity issue on the EAST ICRH antenna were presented, through which the parallel components of electric field and RF-driven sheaths have been greatly improved. The research results expressed in this paper can provide reference for the EAST and China Fusion Engineering Test Reactor (CFETR) ICRH design.